A naturally occurring intertypic porcine rotavirus strain SB-1A was analyzed genetically and serologically. It was shown that this isolate was serotype 4 based on its VP7 and serotype 5 based on its VP3. By analyzing single VP3 gene substitution reassortants, we confirmed that VP3 was as potent in inducing neutralizing antibodies as VP7. The fourth genome segment of porcine rotavirus strains SB-1A and OSU and canine rotavirus strain CU-1 was shown to code for viral hemagglutinin. Human rotavirus strain WI-61 was found to be distinct serotypically from the four known human serotypes.